The present invention relates to a method of manufacturing semiconductor devices, and more particularly to a method of heat treating a semiconductor wafer such as a silicon wafer, first at a high temperature in a non-oxidizing atmosphere, then secondly at a temperature lower than the first mentioned temperature.
In the manufacture of semiconductor devices, including integrated circuits (IC), large scale integration (LSI) and so forth, defects brought about in the processes for fabrication of semiconductor devices and detrimental impurities, which were in the semiconductor wafer or which were mixed into the semiconductor wafer, were found to exert adverse effects on the operational characteristics of the device or device parameters, such as leakage currents, breakdowns, gain and so forth. Such defects and impurities not only shorten the lifetime of the device, but also cause very serious and adverse effects on the device parameters in the process of diffusions of impurities by causing spike diffusions, for example.
In order to prevent occurrence of such defects or to eliminate detrimental impurities, various gettering methods have been introduced into processes for fabrication of semiconductor devices, such as a method of roughening the back side of a semiconductor wafer by sandblast, gettering effected by implantation of ions onto the back side of a semiconductor wafer, and oxidization by hydrogen chloride. Recently, an intrinsic gettering method has been reported in which crystalline defects are created within or inside of a semiconductor wafer for the purpose of utilizing the defects as a sink for gettering.
While the above methods have several merits, it has come to be known, that these methods of gettering exhibit certain defects when applied to the manufacture of semiconductor devices. For example, the method of intentionally damaging the reverse side or back side of the semiconductor wafer by sandblast suffers from the fact that the front side of the semiconductor wafer is subjected to scratches and contaminations. The method of implantation of ions onto the back side of the semiconductor wafer suffers from the same defects as experienced in the sandblast method. The method of oxidizing by means of hydrogen chloride not only involves a lengthy gettering process but also has a further disadvantage in that the semiconductor wafer is warped in the process.
In the intrinsic gettering mentioned above, defects are created in the interior of the semiconductor wafer, and a region devoid of defects, the so-called denuded zone, is formed on the surface of the semiconductor wafer. For this purpose, two methods have been proposed. In the first method, the semiconductor wafer is annealed at a temperature higher than 1000.degree. C. in a non-oxidizing atmosphere.
In a Japanese Patent Application No. 89111 (Filing Date: July 28, 1976) which was published without examination on 14 February 1978 under Publication No. 15764/1978, there is disclosed a gettering method in the fabrication of an oxide layer on an epitaxial layer grown on the front side of a semiconductor substrate. For the purpose of gettering, the semiconductor substrate is first annealed, in a non-oxidizing atmosphere, for one (1) to 20 hours at a temperature higher than 1,000.degree. C. For example, the substrate is annealed in a nitrogen atmosphere or in a vacuum for two (2) hours at 1,200.degree. C. The surface layers in the range of 0.1 to 10 .mu.m of the semiconductor substrate are then removed by etching, for example, and an oxidizing process is then carried out at 1,000.degree. C. in a wet oxygen atmosphere.
A Japanese Patent Application No. 94878 (Filing Date: Aug. 11, 1976), published without examination on Feb. 25, 1978 under Publication No. 20862/1978, discloses another gettering method wherein a semiconductor substrate on which an epitaxial layer has been grown is first heat treated at a temperature in the range of 1,000.degree. to 1,200.degree. C. for forming an oxide layer, then it is annealed in a nonoxidizing atmosphere for four (4) to 30 hours at a temperature in the range of 1,000.degree. to 1,250.degree. C. Thereafter, surface layers in the range of 10 to 50 or 60 .mu.m of the semiconductor substrate are removed.
In the second method, the semiconductor wafer is first annealed in an oxidizing atmosphere at a low temperature below 800.degree. C., then the semiconductor wafer is annealed in a non-oxidizing atmosphere at a high temperature above 1,000.degree. C.
In order to create defects in the interior of the wafer according to the first method, the semiconductor wafer must be such that, when heat treated, oxygen in the crystal may be precipitated. However, with the so-called CZ wafers made by the Czochralski method currently on the market, it has been found that, although they contain a considerable amount of oxygen, oxygen is hardly precipitated. Because of this, it is difficult to create defects in the silicon crystal according to said method as illustrated in FIG. 1A to effect gettering. In this FIG. 1A, which shows a silicon wafer in cross-section, defects 3 are shown in the interior between the front side 1 and back side 2 of the silicon wafer. FIG. 1B is a graphic illustration of the defect density (ordinate) relative to the depth into the silicon wafer (abscissa).
In the second method mentioned above, microdefects are created as shown in FIGS. 2A and 2B by the first annealing step at a low temperature, and then by the subsequent higher temperature annealing step defects capable of gettering are created in the interior of the silicon wafer and denuded zones are formed on the surfaces of the silicon wafer as illustrated in FIGS. 2C and 2D. FIGS. 2A and 2C are views similar to FIG. 1A, and FIGS. 2B and 2D are similar to FIG. 1B, with the same reference numerals referring to the same parts. The second method is effective in overcoming the problem experienced in the first method that is, the difficulty of creating microdefects capable of gettering by a single heat treatment. However, as illustrated in FIG. 2D, a portion of the silicon wafer designated by reference numeral 4 has microdefects which were created near the surfaces by annealing at the low temperature and which cannot be completely eliminated by annealing at the high temperature. Accordingly these microdefects turn out to be macro-defects in subsequent processes, such as the diffusion process, etc. It has been found that portion 4 is very difficult to eliminate and portion 4 remains even if heat treatment is carried out for many hours. It was also found that the defect density in the interior of the silicon wafer becomes low if heat treatment is prolonged in order to eliminate portion 4 completely. It is to be understood that conditions for heat treatment are difficult to select because whether or not the portion 4 can be eliminated is influenced by the crystalline property of the silicon wafer.